WO2009068722A1

WO2009068722A1 - Heat-resistant steel alloy and coiler drum

Info

Publication number: WO2009068722A1
Application number: PCT/FI2007/050642
Authority: WO
Inventors: Osmo Mikkola; Timo Norvasto
Original assignee: Metso Lokomo Steels Oy
Priority date: 2007-11-28
Filing date: 2007-11-28
Publication date: 2009-06-04

Abstract

A - heat- resistant steel alloy for bodies to be manufactured by casting, as well as a coiler drum made of the alloy, which steel alloy contains, in weight percent, a maximum of 0.3% of C, 0.2 to 0.6% of N, 20 to 30% of Cr, 4 to 25% of Ni, 2 to 12% of Mn, 0.3 to 2% of Nb, 0.5 to 3% of Si, the balance being Fe and impurities.

Description

Heat-resistant steel alloy and coiler drum

Field of the invention

The invention relates to a steel alloy according to the preamble of the appended claim 1. The invention also relates to a coiler drum according to the preamble of the appended claim 18.

Background of the invention

- Heat- resistant steels are steels with good mechanical properties and good resistance at high temperatures. The properties of steel are affected, among other things, by varying the composition of the steel alloys used for its manufacture, that is, by varying the alloy elements and their contents. The use of the product to be made of steel will determine the alloy ratios used for manufacturing the steel.

- Heat resistant steel is used, for example, as a material for various rolls and coiler drums used in hot rolling mills. In rolling, a thinner steel strip is made of slabs, such as steel slabs. Steel slabs are rolled between two rotating rolls pressed against each other, and during the rolling, their cross-sectional area is reduced and length increased.

Figure 1 shows schematically a rolling mill 1 comprising rolls 2a and 2b, between which the material to be rolled travels back and forth. The rolls 2a and 2b are in a nip contact with each other. When the material to be rolled travels back and forth through the nip N, its thickness is reduced and it becomes a thinner strip 3. A holding furnace 4 is provided on both sides of the rolling mill. The strip to be rolled is kept at a high temperature by guiding it alternately to the holding furnaces 4. Both furnaces 4 have a rotating cylindrical coiler drum 5, around which the strip is wound and further, when the direction of movement of the strip is changed, unwound from it.

The temperature in the furnaces 4 is about 900 to 1050°C. In the furnaces, the coiler drums 5 are thus subjected not only to a high temperature but also to tension forces caused by the tensioning of the strip against the drum.

A typical coiler drum 5 is shown in Fig. 2. It consists of a hollow cylinder 6 having an outer diameter of at least about 1000 mm, a length of about 2000 to 5000 mm and a wall thickness of about 30 to 150 mm.

The cylinder is normally made by casting of - heat- resistant steel. The cylinder jacket comprises a through hole (not shown in the figure) extending in parallel with the cylinder axis over the length of the jacket. End pieces 7 are provided at both ends of the cylinder. The outer surface of the coiler drum, which is in contact with the strip to be milled, is normally smooth or grooved.

US patent 6,033,626 discloses a - heat- resistant steel alloy which is suitable particularly as a material for rolls and coiler drums used in rolling mills. The alloy is a high alloy that contains little nitrogen.

According to the publication, products made of the alloy do not show spalling. However, a problem with such steels is that at raised temperatures, they show sigma phase formation which decreases the - ductility of the steel. They also have poor corrosion resistance at high temperatures.

Furthermore, a problem with known steels with a small content of nitrogen used as a component, containing less than 0.25 wt-% of nitrogen, is that the high temperature strength of cast products made of them is not the best possible at high temperatures. For example, a coiler drum made of such steel is subject to deformation, that is, change of shape of the drum, as a result of use. This decreases the usability of the drum and may cause that the drum no longer rotates evenly. Moreover, such steel has an increased tendency to form brittle phases, which will result in cracks forming easily in pieces made of the steel when they are used. Summary of the invention

It is thus an aim of the present invention to provide a heat resistant steel alloy with a high yield strength at high temperatures. It is another aim of the invention to provide a coiler drum made of said steel alloy.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The coiler drum according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 18.

The other, dependent claims will present some preferred embodiments of the invention.

The content of nitrogen to be added to the steel alloy according to the invention is substantially higher than in commonly used heat-resistant cast steels. The other components of the alloy and their contents are selected so as to provide an austenitic fire-resistant steel alloy.

The addition of nitrogen has several favourable cross effects on the properties of the steel to be made of the alloy, as well as on its manufacturing costs. When the nitrogen content in the alloy is increased, it is possible to reduce the content of nickel to be added, which is an expensive component. Instead of nickel, it is possible to use manganese. Consequently, the content of manganese, which is already one component of the steel alloy, is increased in the alloy. Manganese is less expensive than nickel, wherein the raw material costs of the steel are reduced. A decrease in the nickel content is known to increase sigma phase formation, but this effect can be eliminated by adding nitrogen. An increase in the manganese content, in turn, has a favourable effect on the formation of the austenitic structure. The increase in the manganese content will also enhance the solubility of nitrogen in austenite. Thanks to the high nitrogen content, products made of the alloy will have a longer service life. Furthermore, the high nitrogen content will raise the yield strength of the products made of the alloy at high temperatures. Experiments have shown that it is even double compared with standard steels. Pilot tests have shown that yield strength has a greater effect on the service life of products than has been previously foreseen. Creep strength, which was previously thought to be a dominant factor on service life, is not a factor as significant for the service life as the yield strength at high temperatures.

The high-temperature yield strength and the creep strength have a significant effect on the deformation resistance of the products at high temperatures. In known steels, deformation of the coiler drum takes place at temperatures of about 1000⁰C. In rolling mill applications it has been found that when a new coiler drum is taken into use, some deformation of the drum will occur soon after the running-in. This is caused by too low a yield strength of the coiler drum. In continued operation, however, the deformation will stop for some time until it begins to occur again. This is caused by the low creep strength of the drum. By means of the invention, it is possible to prevent the deformation of bodies.

The above-defined aims and advantages are achieved with a steel alloy whose chemical composition is, given in weight percents: maximum 0.3% of C

0.2 to 0.6% of N

20 to 30% of Cr

4 to 25% of Ni 2 to 12% of Mn

0.3 to 2% of Nb

0.5 to 3% of Si the balance being Fe and impurities. Brief description of the drawings

Fig. 1 shows schematically a rolling mill according to prior art, and

Fig. 2 shows a coiler drum in a perspective view.

Detailed description of the invention

In this description and in the claims, the term casting refers to the pouring of molten steel alloy into a casting mould, in which it is solidified when cooled. After it has cooled down, the molten alloy will - take the shape defined by the casting mould. The casting mould may be stationary or it may rotate around its axis during the casting. Rotating casting moulds are used for casting bodies of revolution, such as, for example, cylinders, pipes or rings. The term casting steel refers to a steel alloy that is suitable for casting. The term - heat resistant steel, in turn, refers to a steel grade that is resistant to high operating temperatures without scaling in an oxidising atmosphere.

The rolling mill and the coiler drum shown in Figs. 1 and 2 have already been described in connection with the prior art, so that they will not be described in detail any more.

The amounts of components for - heat- resistant casting steel according to the invention, their interaction, and the grounds for the amount of components will be presented in the following. All the alloy percentages are given by weight.

Carbon (C)

Carbon has an effect on the hardness, strength and creep strength of the steel. It is also important for the formation of the austenitic structure. The content of carbon in the alloy should be about 0.3% at a maximum, preferably not more than about 0.2%. However, the alloy should contain at least about 0.1% of carbon. An optimal carbon content in the alloy is about 0.1 to 0.15%. Nitrogen (N)

Nitrogen is also effective on the formation of the austenitic structure. It has also an effect on the high-temperature strength of the steel. Nitrogen can also be used to replace a part of the nickel. A high nitrogen content will increase the high-temperature yield strength of the steel. Thanks to the high nitrogen content, the formation of harmful sigma phase is avoided at raised temperatures. The alloy should contain at least about 0.2% of nitrogen. However, the nitrogen content should be limited to a maximum of about 0.6%, because when the nitrogen content exceeds 0.6%, harmful precipitations are formed in the alloy. The optimal nitrogen content of the alloy is about 0.3 to 0.5%.

Chromium (Cr) Chromium is an important; component that has an effect on the corrosion resistance of the steel at high temperatures. The content of chromium in the alloy should be at least about 20%, preferably at least about 22%. However, the chromium content in the alloy should be about 30% at a maximum, and preferably not more than about 26%. Optimally, the alloy contains about 24% of chromium.

Nickel (Ni)

Nickel acts strongly on the formation of the austenitic structure and prevents, together with nitrogen and manganese, the formation of sigma phase. Nickel is an expensive component in the alloy, and a high nitrogen content will make it possible to replace a part of the nickel, if desired, with manganese, which is a significantly less expensive component. However, the alloy should contain at least about 4% of nickel. Nevertheless, the content of nickel in the alloy should be about 25% at a maximum, preferably not more than about 14%. Optimally, the alloy contains about 4 to 12% of nickel.

Manganese (Mn)

Manganese is also active in the formation of the austenitic structure. It enhances the solubility of nitrogen in austenite, and it is also used to replace nickel. The alloy should contain at least 2%, preferably 4% of manganese. The manganese content is limited to a maximum of about 12%, preferably to 8%. An optimal manganese content in the alloy is about 4 to 8%.

Niobium (Nb)

With carbon, niobium forms niobium-rich carbides. In a thermal treatment of a body made of steel, these form particles that improve the yield strength and the - creep -strength of the body. The content of niobium in the alloy should be at least about 0.3%, preferably at least about 0.7%. Nevertheless, the content of niobium in the alloy should be about 2% at a maximum, preferably not more than about 1.4%. An optimal niobium content in the alloy is about 0.6 to 1%.

Silicon (Si) Silicon is used for desoxydation during the preparation of the melt. The alloy should contain at least about 0.5% of silicon. However, the alloy should not contain more than about 3% of silicon. An optimal silicon content in the alloy is about 0.7 to 1.5%.

In addition to the components mentioned above, the steel alloy does not contain other deliberately added components than iron (Fe). In addition, the alloy may contain small amounts of impurities which do not affect the properties of the steel.

Description of the tests carried out and the results obtained

The examined steels were made in melt batches in laboratory scale. The chemical composition of the examined steels in weight percents is presented in Table 1. In addition to the components presented in the table, the steel contained only iron and impurities. Sample 1 is a commercially available steel used as a reference sample.

In addition, Table 1 shows the 0.2 proof stress of the steel samples tested at the temperature of 1000⁰C. Of the examined steels, test rods were cast which were subjected to thermal treatment before the tests. Table 1.

Com osition of the examined tests, wt-%

As can be seen from the result of the tests on tensile strength,- the 0.2 proof stress of the samples having the composition of the invention is significantly higher than that of the reference sample.

The steel alloy according to the invention is particularly suitable for use in cast products, such as coiler drums applied in rolling mills. Particular benefits are obtained by the coiler drums from the material that gives them both high-temperature yield resistance and high-temperature creep strength.

The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims.

Claims

1. A - heat resistant steel alloy for bodies to be manufactured by casting, characterized in that the chemical composition of the alloy is, in weight percent: maximum 0.3% of C

0.2 to 0.6% of N

20 to 30% of Cr

4 to 25% of Ni 2 to 12% of Mn

0.3 to 2% of Nb

0.5 to 3% of Si the balance being Fe and impurities.

2. The steel alloy according to claim 1 , characterized in that it contains a maximum of 0.2% of C, preferably not more than 0.15% of C.

3. The steel alloy according to claim 1 or 2, characterized in that it contains at least 0.1% of C.

4. The steel alloy according to any of the preceding claims 1 to 3, characterized in that it contains at least 0.3% of N.

5. The steel alloy according to any of the preceding claims 1 to 4, characterized in that it contains not more than 0.5% of N.

6. The steel alloy according to any of the preceding claims 1 to 5, characterized in that it contains at least 22% of Cr.

7. The steel alloy according to any of the preceding claims 1 to 6, characterized in that it contains not more than 26% of Cr.

8. The steel alloy according to claim 6 or 7, characterized in that it contains about 24% of Cr.

9. The steel alloy according to any of the preceding claims 1 to 8, characterized in that it contains a maximum of 14% of Ni, preferably not more than about 12% of Ni.

10. The steel alloy according to any of the preceding claims 1 to 9, characterized in that it contains at least 4% of Mn.

11. The steel alloy according to any of the preceding claims 1 to 10, characterized in that it contains not more than 8% of Mn.

12. The steel alloy according to any of the preceding claims 1 to 11 , characterized in that it contains at least 0.6% of Nb.

13. The steel alloy according to any of the preceding claims 1 to 12, characterized in that it contains at least 0.7% of Nb.

14. The steel alloy according to any of the preceding claims 1 to 13, characterized in that it contains a maximum of 1.4% of Nb, preferably not more than 1 % of Nb.

15. The steel alloy according to any of the preceding claims 1 to 14, characterized in that it contains at least 0.7% of Si.

16. The steel alloy according to any of the preceding claims 1 to 15, characterized in that it contains not more than 1.5% of Si.

17. The steel alloy according to claim 1 , characterized in that the body to be manufactured by casting is a roll or- a coiler drum to be used in a rolling mill.

18. A coiler drum made by casting of a fire-resistant steel alloy, characterized in that the chemical composition of the alloy is, in weight percent: maximum 0.3% of C 0.2 to 0.6% of N

20 to 30% of Cr 4 to 25% of Ni 2 to 12% of Mn 0.3 to 2% of Nb 0.5 to 3% of Si the balance being Fe and impurities.

19. The coiler drum according to claim 18, characterized in that the steel alloy used in the manufacture of the coiler drum contains a maximum of 0.2% of C, preferably a maximum of 0.15% of C.

20. The steel alloy according to claim 18 or 19, characterized in that the steel alloy used in the manufacture of the coiler drum contains at least 0.1% of C.

21. The coiler drum according to any of the preceding claims 18 to 20, characterized in that the steel alloy used in the manufacture of the coiler drum contains at least 0.3% of N.

22. The coiler drum according to any of the preceding claims 18 to 21 , characterized in that the steel alloy used in the manufacture of the coiler drum contains not more than 0.5% of N.

23. The coiler drum according to any of the preceding claims 18 to 22, characterized in that the steel alloy used in the manufacture of the coiler drum contains at least 22% of Cr.

24. The coiler drum according to any of the preceding claims 18 to 23, characterized in that the steel alloy used in the manufacture of the coiler drum contains not more than 26% of Cr.

25. The steel alloy according to claim 23 or 24, characterized in that the steel alloy used in the manufacture of the coiler drum contains about 24% of Cr.

26. The coiler drum according to any of the preceding claims 18 to 25, characterized in that the steel alloy used in the manufacture of the coiler drum contains a maximum of 14% of Ni, preferably not more than 12% of Ni.

27. The coiler drum according to any of the preceding claims 18 to 26, characterized in that the steel alloy used in the manufacture of the coiler drum contains at least 4% of Mn.

28. The coiler drum according to any of the preceding claims 18 to 27, characterized in that the steel alloy used in the manufacture of the coiler drum contains not more than 8% of Mn.

29. The coiler drum according to any of the preceding claims 18 to 28, characterized in that the steel alloy used in the manufacture of the coiler drum contains at least 0.6% of Nb.

30. The coiler drum according to any of the preceding claims 18 to 29, characterized in that it contains at least 0.7% of Nb.

31. The coiler drum according to any of the preceding claims 18 to 30, characterized in that the steel alloy used in the manufacture of the coiler drum contains a maximum of 1.4% of Nb, preferably not more than 1 % of Nb.

32. The coiler drum according to any of the preceding claims 18 to 31 , characterized in that the steel alloy used in the manufacture of the coiler drum contains at least 0.7% of Si.

33. The coiler drum according to any of the preceding claims 18 to 32, characterized in that the steel alloy used in the manufacture of the coiler drum contains not more than 1.5% of Si.